Parahelicoprion

Parahelicoprion Temporal range: Early Permian ~
A speculative life restoration of Parahelicoprion clerci, with upper jaw and whorl reconstructed in accordance with Sarcoprion
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Holocephali
Order:	†Eugeneodontida
Family:	†Helicoprionidae
Genus:	†Parahelicoprion Karpinsky, 1924
Type species
Helicoprion clerci Karpinsky, 1916
Species
P. clerci Karpinsky, 1916 P. mariosuarezi Merino-Rodo & Janvier, 1986

Parahelicoprion ('nearly coiled saw') is an extinct genus of shark-like cartilaginous fish known from the early Permian of the Ural Mountains of Russia and the Copacabana Formation of Bolivia. Members of the genus possessed a row of large, serrated tooth crowns on the lower jaw, called a tooth whorl, and are considered members of the order Eugeneodontida. The genus name refers to relation with Helicoprion, another eugeneodont from the Ural Mountains which also bore a curved tooth whorl along the midline of the lower jaw. Two species of Parahelicoprion are assigned; the Russian P. clerci and the Bolivian P. mariosuarezi.

The holotype of Parahelicoprion is poorly preserved, consisting of only six partial tooth crowns, and that of the Bolivian P. mariosuarezi consists of only nine crowns, all of which missing their cutting edges. The fragmentary nature of the dentition renders estimates of both the nature of the complete whorl and the body size of the animal highly speculative. When first described, P. clerci was considered a member of the genus Helicoprion, although its initial describer, Alexander Karpinsky, later separated the type species into its own genus. It has since once more been suggested that this genus does represent a junior synonym of Helicoprion.

Classification and description

Parahelicoprion has been distinguished from other members of its family, the Helicoprionidae, by the shape of its tooth crowns. The helicoprionids (sometimes referred to as agassizodontids) are as a group defined by a series of blade-like crowns which emerge from a single fused compound root, known as a tooth whorl, which is angled forward and positioned along the symphysis of the lower jaw. Members of the group also possessed a laterally positioned pavement of flattened crushing teeth, although these have not been identified in either species of Parahelicoprion. Other aspects of the group's jaws and skull are only rarely preserved, and nothing is known of their postcranial anatomy.

The tooth whorl of the potential close relative Sarcoprion edax

In P. clerci, the known crowns are lined with deep serrations, and the single preserved upper edge is broad, flattened, and triangular. In P. mariosuarezi these deep serrations are apparently absent across most of the crown, but several large, pointed denticles do protrude along the lower tips. This species also preserves a single denticle-bearing parasymphyseal tooth along the edge of the whorl. While in P. clerci the whorl is too incomplete for the structure to be gauged, the holotype of P. pariosuarezi shows the smallest crown was positioned anteriorly, at the very front of the preserved portion of the whorl, and is significantly smaller than the next crown in the sequence. The structure of the dentition in Parahelicoprion has been likened to that of Sarcoprion, another helicoprionid for which a more complete tooth whorl is known. The full extent of Parahelicoprion's tooth whorl is unknown.

Paleobiology

Parahelicoprion inhabited marine environments, and is thought to have been a nektonic carnivore. While the genus is considered to be among the largest of the eugeneodonts, estimates of its body size have been described as "controversial".